Hitherto, a semiconductor device comprising an insulating ceramic board, a metal circuit layer provided on one surface of the board, a heat radiating metal plate fixed on the other surface of the board, and semiconductor tips for controlling an electric voltage and current soldered on the metal circuit layer has been proposed. One or more semiconductor devices mentioned above are bonded with a soldering material on one surface of a metal base plate ( heat sink ) or on a composite member, and a heat releasing fin or fins are mounted through thermal conductive greases on the other surface of the metal base plate.
However, the above conventional semiconductor device has following problems. {circle around (1)} The bonding with a lead solder is used mainly, so that it is required to develop a new soldering material or soldering method in order to realize a lead free soldering method. {circle around (2)} The bonding with a lead solder is used mainly, so that there is a loss in heat transmission due to the lead in the soldering material. {circle around (3)} The bonding with a lead solder is used mainly, so that there is a risk to reduce the yield rate of the manufacturing due to the voids of lead solder.
(4) The thermal conductivity of a thermal conductive grease is low remarkably. That is, the thermal conductivity of the thermal conductive grease on the market is several W/m·K at most.
The above problems (1) to (3) can be solved by using an insulating ceramic substrate bonded directly to a base plate. However, the problem (4) cannot be solved because no materials of higher thermal conductivity can be available on the market.
Japanese Patent Application Laid-Open No. 363052/92 discloses a semiconductor device, wherein heat radiating fins are directly bonded on a ceramic board by a brazing method. However, if the fin of aluminum large in coefficient of thermal expansion and volume is directly bonded on the thin ceramic board small in coefficient of thermal expansion, the ceramic board would be separated from the aluminum fin or the ceramic board would be damaged by the stress due to the difference in coefficient of thermal expansion between the fin and the ceramic board, when they are heated and cooled alternately and frequently and elevated in temperature as like as in the actual power module operation.
The task of the present invention is to solve the above problems and defects.